Automated inspection of solder joints, especially those used on printed circuit boards, has become increasingly important with the proliferation of automatic electronic assembly technology. Human visual inspection is becoming less cost effective and more unreliable because of the time consuming microscopic inspection required for the typically thousands of solder joints per board. A primary feature of solder joint quality is the shape of the joint surface. Therefore the ability to automate the measurement of solder joint shapes and detect defects through shape is a significant requirement of a fully automated system. Current progress toward automated inspection includes development of optical non-contact inspection systems capable of very accurate measurements but these systems are fairly complex.
The pending patent application referenced in the CROSS-REFERENCE TO RELATED APPLICATION section entitled SOLDER JOINT INSPECTION SYSTEM addresses an optical system for determining the orientation of a number of points on the surface of a specular object, such as a solder joint, and utilizing known surface features of the object along with the known orientations to approximately reconstruct the shape of the object.
The pending patent application referenced in the CROSS-REFERENCE TO RELATED APPLICATION section entitled A DEVICE FOR EXTRACTING SHAPE AND REFLECTANCE OF THREE DIMENSIONAL SURFACES addresses an optical system for object surface shape and reflectance extraction utilizing extended light sources, not point light sources.
U.S. Pat. No. 4,695,163 issued on Sept. 22, 1987 to Ronald A. Schachar entitled "Method and Apparatus for Determining Surface Shapes Using Reflected Laser Light" discloses a method and apparatus for determining the surface shape of an object by scanning the object with a light beam incrementally movable in a linear and angular direction. The light beam must be physically moved and directed across the surface of the object a number of times and at a multitude of angles so that light detectors may provide adequate accurate information for a determination of the surface shape. Using light beams which cover only a portion at a time of the object to be inspected, this method and apparatus provides precise information to determine surface orientations but becomes cumbersome when only an approximate profile is desired.
U.S. Pat. No. 4,508,452 issued on Apr. 2, 1985 to Paul L. DiMatteo, Joseph A. Ross, and Howard K. Stern entitled "Arrangement for Sensing the Characteristic of a Surface and Determining the Position of Points Thereon" discloses a method and arrangement for optical inspection utilizing a projector illuminating a series of predetermined sections on an object and simultaneously a camera moving about the object taking multiple photographs for subsequent analysis. The entire surface of the object is inspected and accurately mapped. No prior knowledge of the shape of the object is needed for this approach.
U.S. Pat. No. 4,427,880 issued on Jan. 24, 1984 to Takeo Kanade and Haruhiko Asada entitled "Non-Contact Visual Proximity Sensing Apparatus" teaches an apparatus for determining the location and orientation of an object utilizing a plurality of light sources spaced apart in a pattern. The light sources are directed at a symmetrical object and the reflected beams impinge on the surface of a light sensitive transducer. The pattern of these reflected beams is then analyzed to determine the location and relative orientation of the object. Note this technique does not reveal any details about the surface geometry but only the orientations and locations of an object whose geometry is already known.
U.S. Pat. No. 4,791,482 issued on Dec. 13, 1988 to Robert F. Barry and Samuel Kang entitled "Object Locating System" teaches the generation of Gaussian images for comparison between reference objects and objects to be identified. See column 7, line through column 8, line 22.
FIG. 1A shows one lead 10 from a typical surface mounted flatpack semiconductor chip 11 attached with solder shown generally by 12 to a solder pad 13 on a printed circuit board 14. FIG. 1B shows the top view of this arrangement while FIG. 1C shows a cross-section view "1C--1C". For assessment of the quality of the solder joint 12 for a flatpack chip 11, the relevant part of the lead 10 for inspection is the underside of the foot 15 portion of the lead 10. Because the foot 15 is positioned flat on the solder pad 13, a small amount of solder is sufficient to make a proper connection between the foot 15 and the solder pad 13. Note the meniscus 16 of solder that has been displaced from under the foot 15 to the sides and back of the foot 15. The portion of the meniscus 16 to the front of the foot 15 is the toe 17. The portion to the sides is the shoulder 18, while that portion of the meniscus to the rear of the foot 15 is the heel fillet 19. The wetting of the lead can be assessed at its true worth from the solder meniscus, however small, at all four sides of the foot 15. Further requirements on the amount of solder for flatpacks is usually not necessary. Note that further information on the assessment of solder joints may be found in "Soldering in Electronics" by R. J. Klein Wassink. FIG. 1C highlights critical features of a typical solder joint such as the toe 17, the shoulder 18, and the heel fillet 19 on a lead 10 of a surface mounted chip 11 on a printed circuit board 14. Note again the most critical solder is not apparent with visual inspection because it occurs between the bottom of the foot 15 and the top of the solder pad 13. The meniscus shown by the toe 17, shoulder 18, and heel fillet 19 is indicative of the quality of that unseen solder joint. By examining the shape of the solder meniscus and comparing this to known acceptable shapes, it is possible to determine the acceptability of the solder joint. Typically, in order to determine the acceptability of a solder joint, the shape of the joint is determined and that shape compared with the shapes of acceptable solder joints. Because a large multitude of shapes may be acceptable shapes, this approach becomes fairly involved and cumbersome.
It is the object of this invention to provide a method and system for determining profile information of solder joints, utilizing reflections from their specular surfaces. Using optical scanning along the surface of a solder joint, it is desired to generate a profile approximating that of the solder joint.
Another object of the present invention is to provide an improved method and system for surface profile determination of solder joints using a selective optical sensing such that safe, low power optical sources may be used.
Another object of the present invention is to reduce the number of data points required to determine the surface geometry of the solder joint by utilizing a priori knowledge about the salient features of the solder joint to predict the behavior of the surface in regions where data was not acquired.
Yet another object of the present invention is to compare the surface features of the object, i.e. solder joint, with typical solder joint surface features to determine joint integrity because the shape of the solder joint reveals the quality of the joint.
Still another object of the present invention is to provide a method for evaluating the solder joint based solely on the surface features of the joint such that joint reconstruction is unnecessary.